1. Field of the Invention
The invention relates to a liquid crystal compound, a liquid crystal composition and a liquid crystal display device. More specifically, it relates to a difluorobenzene derivative having alkenyl and cyclohexenyl, a liquid crystal composition having a nematic phase and including the compound, and a liquid crystal display device including the composition.
2. Related Art
A liquid crystal display device, which is represented by a liquid crystal display panel and a liquid crystal display module, utilizes optical anisotropy and dielectric anisotropy of a liquid crystal compound (which is a generic term for a compound having a liquid crystal phase, such as a nematic phase, a smectic phase and so forth, and also for a compound having no liquid crystal phase but being useful as a component of a composition). As an operation mode of a liquid crystal display device, various modes have been known, such as a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, a bistable twisted nematic (BTN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode, and so forth.
Among these modes, an ECB mode, an IPS mode, a VA mode and so forth are operation modes utilizing vertical orientation property of a liquid crystal molecule, and in particular, it has been known that an IPS mode and a VA mode can eliminate a narrow viewing angle, which is a defect of the conventional modes, such as a TN mode and an STN mode.
As a component of a liquid crystal composition having a negative dielectric anisotropy, which can be used in a liquid crystal display device of these modes, various kinds of liquid crystal compounds, in which hydrogen on a benzene ring is replaced by fluorine, have been investigated (as described, for example, in JP H02-4725 A/1990, JP 2000-53602 A, JP H10-237075 A/1998, JP H02-4723 A/1990, JP 2002-193853 A, WO 89/08633, WO 89/08687 and EP 1333017).
For example, a compound (A), in which hydrogen on a benzene ring is replaced by fluorine, is studied in JP H02-4725 A/1990, but the compound has a small optical anisotropy.
A compound (B), in which hydrogen on a benzene ring is replaced by fluorine, having alkenyl is studied in JP 2000-53602 A, but the compound does not have a sufficiently large optical anisotropy.
A compound (C), in which hydrogen on a benzene ring is replaced by fluorine, having cyclohexenylene and oxabicyclopentane is studied in JP H10-237075 A/1998, but the mesophase range where the compound exhibits liquid crystallinity is significantly narrow, and a composition including the compound does not show a high clearing point.
A compound (D) and a compound (E), in which hydrogen on a benzene ring is replaced by fluorine, having cyclohexenylene have been reported, for example, in JP H02-4723 A/1990 and JP 2002-193853 A, but the compounds exhibit poor compatibility with other liquid crystal compounds in a low temperature range.
WO 89/08633, WO 89/08687 and EP 1333017 report compounds, in which the hydrogen on a benzene ring is replaced by fluorine, having cyclohexenylene, are disclosed as an intermediate, but the compounds are utilized only as a raw material or an intermediate, and furthermore, no compound having both alkenyl and cyclohexenylene simultaneously has not been studied.

Accordingly, a liquid crystal display device having an operation mode, such as an IPS mode or a VA mode, still has problems as a display device compared to CRT, and, for example, needs to be improved in response time and contrast and decreased in driving voltage.
The display device driven in an IPS mode or a VA mode mainly includes a liquid crystal composition having a negative dielectric anisotropy, and in order to improve the aforementioned properties, a liquid crystal compound included in the liquid crystal composition necessarily has the following properties (1) to (8):
(1) The compound is chemically stable and physically stable;
(2) The compound has a high clearing point (transition temperature from a liquid crystal phase to an isotropic phase);
(3) The compound has a low minimum temperature of a liquid crystal phase (such as a nematic phase and a smectic phase), and particularly has a low minimum temperature of a nematic phase;
(4) The compound has a low viscosity;
(5) The compound has a suitable optical anisotropy;
(6) The compound has a suitable negative dielectric anisotropy;
(7) The compound has a suitable elastic constant K33 (K33: bend elastic constant); and
(8) The compound is excellent in compatibility with other liquid crystal compounds.
In the case where a composition including a liquid crystal compound that is chemically and physically stable as in (1) is used in a display device, the voltage holding ratio can be increased.
In the case where a composition including a liquid crystal compound having a high clearing point or a low minimum temperature of a liquid crystal phase as in (2) and (3) is used, a temperature range of the nematic phase can be enhanced, and a display device can be used in a wide temperature range.
In the case where a composition including a compound having a small viscosity as in (4) or a large elastic constant K33 as in (7) is used in a display device, the response time can be improved. In the case where a composition including a compound having a suitable optical anisotropy as in (5) is used in a display device, the contrast of the display device can be improved. Liquid crystal compounds having optical anisotropy varying over a wide range are necessary depending on design of a display device. In recent years, a method has been studied in which the cell thickness is decreased to improve the response time and, accordingly, a composition having a large optical anisotropy is desired.
Furthermore, in the case where a liquid crystal compound has a large negative dielectric anisotropy, a liquid crystal composition including the compound can have a low threshold voltage, and accordingly, a display device using a composition including a suitable negative dielectric anisotropy as in (6) can have a low driving voltage and a small electric power consumption. Furthermore, in the case where a composition including a compound having a small elastic constant K33 as in (7) is used in a display device, the display device can have a low driving voltage and a small electric power consumption.
A liquid crystal compound is generally used as a composition by mixing with other various liquid crystal compounds for obtaining properties that cannot be exhibited with a single compound. Accordingly, a liquid crystal compound used in a display device preferably has good compatibility with other liquid crystal compounds as in (8). Furthermore, a display device may be used over a wide temperature range including a temperature below freezing point, and therefore, the compound preferably exhibits good compatibility in low temperature ranges.